Recently, considerable progress has been made in the generation of high power pulses for applications to wide-band impulse transmitters. For example, a pulsed laser beam has been used to render material at the end of a charged transmission line conductive so that it would immediately discharge and cause a traveling wave to flow through that end of the line to an antenna coupled to it. Such a transmission line structure acted as both an energy storage device and a switch.
Although coaxial and microstrip transmission lines have been used in this capacity in the past, the art has settled on using radial transmission lines structures. This is mainly due to the fact that the traveling waves converge at the center of such radial transmission line structures, and thus produce pulses of higher energy. Consequently, devices comprised of such structures require less charge to achieve the same output level as microstrip devices, and thus provide greater device efficiency and longer device life. These and other properties that make radial transmission line structures more desirable than other structures are described in an article entitled "Monolithic, photoconductive impulse generator using a GaAs wafer," authored by the present inventors was published in "Applied Physics Letters 58 (24)," on Jun. 17, 1991.
A monolithic photoconductive impulse generator utilizing such a radial transmission line structure was disclosed in the inventors copending application, Docket No. CECOM 4722, entitled "Pulse Sharpening Using An Optical pulse," and is incorporated herein by reference. This device utilizes an optical means to control the discharge of a radial transmission line structure having the properties described above to increase the radiated bandwidth well above one gigahertz.
Basically, the disclosed optical means comprises a laser beam that triggers both the discharge of the radial transmission storage structure and the termination of that discharge at some predetermined time later. The optical means further comprises a laser light source, a beam splitter, and two fiber cables having predetermined lengths which are different from each other. The beam splitter splits the beam emitted from the laser light source and directs the first beam into the first and shorter fiber cable, and the second beam into the second and longer fiber cable. For this reason, the second beam travels a longer optical path than the first beam such that a predetermined delay is created between the cable paths. The first beam is utilized to trigger the discharge of the radial transmission line, whereas the second beam is utilized to trigger the termination of the discharge. Consequently, the energy discharge profile is sharpened and the radiated pulse bandwidth is widened.
Although the above pulse sharpening device improves the overall efficiency and performance over the prior art, the device does not fully utilize the geometry of the radial transmission line structure, as described above. In particular, the device does not fully utilize the voltage gain nor does it fully provide the operating efficiency that such a structure is capable of providing. Consequently, those skilled in the art greatly desire such a device that fully utilizes the geometry of radial transmission line structures, and thus substantially improves the radiation bandwidth, voltage gain, operating efficiency, and operating lifetime of the device without substantially adding to its cost or size.